1. Field of the Invention
This invention relates to a rubber composition having improved mechanical properties, particularly an excellent compressive strength.
This invention also relates to a golf ball which comprises the rubber composition mentioned above and has excellent durability, bounce impact elasticity (flight characteristics) and excellent feeling upon hitting.
This invention further relates to a rubber composition particularly suitable for a cover layer of a golf ball.
2. Description of the Prior Art
Rubber compositions, unlike plastics, exhibit viscoelasticity and, by the virture of this property, have found utility in a wide variety of fields such as civil engineering, the machine industry, electrical industry and chemical industry. In recent years, rubber compositions have been required to possess high hardness, high elasticity, high strength, extremely high compressive strength and excellent wear resistance to meet diversified needs by the same token that high-strength plastics had been developed. Further, those rubber composition are in most cases required to possess at least two of such highly improved properties simultaneously.
Golf balls, for example, are required to ensure flight stability, long flight distance characteristics and the like. The rubber compositions used in golf balls, therefore, are required to have well controlled properties such as bounce impact elasticity, compressive strength and hardness, and various rubber compositions having improved properties have been proposed.
For example, a golf ball composed of a composition containing metal salt monomers of an .alpha.,.beta.-monoethylenically unsaturated carboxylic acid has been proposed. In this type of golf ball, the monomers had been grafted on the main chain of the rubber component of polybutadiene in the presence of a free radical initiator to function as a co-cross-linking agent, and thereby hardness is imparted to the ball. However, the sufficient durability and bounce impact elasticity have not been obtained with such golf ball. The hardness of a golf ball can be controlled by the amount of the cross-linking agent, however, increased amount of the cross-linking agent would cause a decline of the bounce impact elasticity.
For the purpose of the improvement of hardness and wear resistance of rubber compositions, various compositions incorporated with finely divided inorganic material have been also proposed, however, the decrease of the elasticity and decrease of compression fracture strength are inevitably caused without exception. The incorporation of long staple fibers has been proved to be effective for enhancing the strength of the composition, however, the incorporation of such fibers impairs the surface smoothness and also fails to produce any significant improvement in elasticity and hardness.
Based on the technical concept of those conventional methods mentioned above, use of fine fibers in a rubber composition was proposed, however, they exhibit poor wettability to a rubber material to cause slippage between their surfaces and the rubber material. Because of the slippage, it is difficult to obtain the desired properties in such composite material, though the fine fibers can be dispersed uniformly in a given rubber composition. Therefore, a surface treatment of the fibers has often remained as a problem to be solved. It is well known that hydrolyzable silyl compounds are effective surface treating agents, but the compound(s) must be selected to suit the particular kind of a rubber component used in the composition and the surface treatment with the compound(s) requires highly advanced skill.
We have already found that a sulfur-containing silyl compound possesses an excellent affinity to rubber components and serves as an excellent agent for the surface treatment mentioned above. However, heretofore such sulfur-containing silyl compounds have been invariably directed to sulfur vulcanizing systems, and it has been believed that the sulfur-containing silyl compound cannot be used for a non-sulfur vulcanizing system since it has been supposed that the non-sulfur vulcanization would be inhibited with the sulfur-containing silyl compound judging from its vulcanization mechanism.